Gene Therapy for Muscular Dystrophy
Duchenne Muscular Dystrophy Muscular dystrophy is a group of muscle diseases characterized by skeletal muscle weakness due to improper formation and death of muscle proteins and tissues. Common symptoms are due to progressive muscular wasting but include poor balance, scoliosis, inability to walk, respiratory difficulty, muscle spasms and more debilitating results (Muscular 2013.) Duchenne (named after Guillame Duchenne) is the most severe form of MD that affects 1 in 3,600 boys in the US and is characterized by severe muscle degradation and eventually death. Patients will first show symptoms around age 5, they will be unable to walk unassisted by age 12 and die between age 20 and 40. The disease is an X-linked recessive disorder resulting from a mutation within the 2.3 Mb dystrophin gene which is the largest in the human genome (New 2010.) DMD mutations cause a malformation of the dystrophin protein inhibiting the proper formation of the dystroglycan complex on the cell membrane (Gene 2013.) The C-terminus of Dystrophin adheres actin filaments to the ECM using the sarcoglycan/dystroglycan compexes. These transmembrane complexes are crucial for membrane stability, force transduction and muscle fiber attachment (Gene 2013.) Alleviation of symptoms for this disease via gene therapy will require efficient delivery of a functional protein cassette into a massive gene that affects all of the striated muscles of the body.. Read-through of Premature Nonsense Mutations Though only 15% of DMD mutations are caused by substitution mutations resulting in a mis-placed stop codon, a new treatment using aminoglycoside antibiotics has been formulated to remedy this problem. Gentamicin has been shown to cause cells to ignore both normal and mutant stop codons, but results suggest it is more effective on mutant stop codons due to their location within the coding sequence. The sequence of the stop codon (TGA, TAA, TAG) along with the gentamicin isomer has also been a contributing factor to antibiotic effectiveness. 174 human clinical trials have been done using the aminoglycoside PTC-124 and they have yielded positive results. Three sample groups were selected and a high PTC-124, low PTC-124 and control group were given either PTC or a placebo in a double blind study format. Results showed increased production of the dystrophin protein up to 15-20% in striated muscle cells (New 2010) in the high PTC-124 group, though results featured a large standard deviation. This was attributed to nonsense mediated mRNA decay, small sample size and a multitude of other factors. Future research will yield if a balance between PTC-124 efficiency and toxicity can be reached. Antisense-mediated Exon Skipping The mutation (whether it be deletion, insertion or base change) that causes DMD is found in a coding exon. Therefore the potential of DNA splicing to fix the mutation or reading frame error could allow formation of a "partially functional protein." Antisense oligonucleotides are bound to target sequences to hide the exon from splicing machinery and alter the splicing frame. Intravascular oligonucleotide injection has shown success up to 35%, but this was only seen in isolated areas of the body. The largest issue that arises is that muscle mass makes up approximately 30-40% of the body (Gene 2010) and therefore the antisense-mediated exon skipping would have to be enabled throughout. This type of gene therapy may be used to isolate muscle cells that contribute to autocrine functions such as the beating of the heart, the inspiration/expiration of the lungs and the movement of the GI tract. Works Cited #Gene Theraphy of Muscular Dystrophy . 2013. Human Molecular Genetics. Oxford Journals. Vol. 11 Issue 20 pg. 2355-2362 #Muscular dystrophy . (2013, October 18). In Wikipedia, The Free Encyclopedia. Retreived 21:01, October 21, 2013 #New insights in gene-derived therapy (2010) Aartsma-Rus, A., den Dunnen, J. T. and van Ommen, G.-J. B. : the example of Duchenne muscular dystrophy. Annals of the New York Academy of Sciences, 1214: 199–212. doi: 10.1111/j.1749-6632.2010.05836.